It is well known that high energy radiation can cause appropriate substances to luminesce. Substances showing the phenomenon of luminescence under the influence of high energy radiation are called phosphors or scintillators.
A well known use of phosphors is in the production of X-ray images. In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In practice the light emitted imagewise by said screen irradiates a contacting photographic silver halide emulsion layer film which after exposure is developed to form therein a silver image in conformity with the X-ray image.
For use in common medical radiography the X-ray film comprises a transparent film support, coated on both sides with a silver halide emulsion layer. During the X-ray irradiation said film is arranged in a cassette between two X-ray conversion screens each of them making contact with its corresponding silver halide emulsion layer.
According to another method of recording and reproducing an X-ray pattern disclosed e.g. in U.S. Pat. No. 3,859,527 a special type of phosphor is used, known as a photostimulable phosphor, which being incorporated in a panel is exposed to incident pattern-wise modulated X-rays and as a result thereof temporarily stores therein energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are processable to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam.
As described in U.S. Pat. No. 4,239,968 europium-doped barium fluorohalides are particularly useful for application as stimulable phosphors for their high sensitivity to stimulating light of a He--Ne laser beam (633 nm), ruby laser beam (694 nm) and YAG laser beam (1064 nm), the optimum of stimulation being in the range of 500 to 700 nm. The light emitted upon stimulation, called stimulated light is situated in the wavelength range of 350 to 450 nm with its main peak at 390 nm (ref. the periodical Radiology, September 1983, p. 834).
As described in said periodical the imaging plate containing the stimulable phosphor can be used repeatedly to store X-ray images simply by flooding it with light to erase the residual energy it contains.
For use as photostimulable phosphors, europium-doped barium fluorohalides as described in, e.g., U.S. Pat. No. 4,239,968, halosilicates or halogermanates as described in, e.g., EP-B 382 295 and alkalimetal/alkaline earth metal halides as described in, e.g., U.S. Pat. No. 5,028,509 are preferred. Each of these classes of compounds do have their own advantage and disadvantages. As can be learned from DE-OS 3,347,207 europium-doped barium fluorohalides are chemically not stable and are more particularly sensitive to moisture which according to experiments affects their fluorescence power.
The trough-put of a digital radiography system, based on the use of storage phosphors, strongly depends on the speed with which the image on the storage phosphor plate can be scanned without adverse effects on image quality. This speed is limited by the decay-time of the stimulated luminescence. The decay-time for the best known and mostly used storage phosphors (e.g. europium-doped barium fluorohalides) is about 500 ns. Therefore the need still exists for storage phosphors with a decay-time that is substantially shorter than 500 ns.
The image quality that is produced by a conventional as well as by a digital radiographic system, mainly depends on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality. This means that the lower the ratio binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimum sharpness can thus be obtained when "single crystal" screens (i.e. screens without any binder) are used. Such screen can be produced, e.g., by vacuum deposition of phosphor material on a substrate. However, this production method can not be used to produce high quality screens with every arbitrary phosphor available. The mentioned production method leads to the best results when phosphor crystals with high crystal symmetry are used. Phosphor having complicated crystal structures as, e.g., alkaline earth fluorohalides, tend to decompose (partially) under vacuum deposition and the production of screens by vacuum deposition while using phosphors with complicated crystal structure is quasi impossible and leads to sub-optimal results.
From the above it is clear that the need for a novel class of compounds useful as photostimulable phosphor and that combine the advantages of the prior art phosphors with a lesser degree of disadvantages is still real.
Also in classical radiography various compounds are used as phosphors, but also in this sector the need for new compounds still exists, because in this sector the need for higher speed, i.e. lower X-ray dosis for the patient, combined with high sharpness and low noise continues to exist.